The present invention relates to a process for controlling the (co)polymerisation of olefins in a continuous polymerisation reactor. Preferably, the present invention relates to a process for controlling the gas-phase (co)polymerisation of ethylene in a continuous fluidised bed reactor.
Processes for the (co)polymerisation of olefins in the gas phase are well known in the art. Such processes can be conducted for example by introducing the gaseous monomer (and comonomer) into a stirred and/or gas fluidised bed comprising polyolefin and a catalyst for the polymerisation.
In the gas fluidised bed polymerisation of olefins, the polymerisation is conducted in a fluidised state by means of an ascending gas stream comprising the gaseous reaction monomer. The start-up of such a polymerisation generally employs a bed of polymer particles similar to the polymer that it is desired to manufacture. During the course of polymerisation, fresh polymer is generated by the catalytic polymerisation of the monomer, and polymer product is withdrawn to maintain the bed at more or less constant volume. An industrially favoured process employs a fluidisation grid to distribute the fluidising gas to the bed, and to act as a support for the bed when the supply of gas is cut off. The polymer produced is generally withdrawn from the reactor via a discharge conduit arranged in the lower portion of the reactor, near the fluidisation grid. The fluidised bed consists in a bed of growing polymer particles. This bed is maintained in a fluidised condition by the continuous upward flow from the base of the reactor of a fluidising gas.
The polymerisation of olefins is an exothermic reaction and it is therefore necessary to provide means to cool the bed to remove the heat of polymerisation. In the absence of such cooling the bed would increase in temperature and, for example, the catalyst becomes inactive or the bed commences to fuse. In the fluidised bed polymerisation of olefins, the preferred method for removing the heat of polymerisation is by supplying to the polymerisation reactor a gas, the fluidising gas, which is at a temperature lower than the desired polymerisation temperature, passing the gas through the fluidised bed to conduct away the heat of polymerisation, removing the gas from the reactor and cooling it by passage through an external heat exchanger, and recycling it to the bed. The temperature of the recycle gas can be adjusted in the heat exchanger to maintain the fluidised bed at the desired polymerisation temperature. In this method of polymerising alpha olefins, the recycle gas generally comprises the monomer and comonomer olefins, optionally together with, for example, an inert diluent gas such as nitrogen or a gaseous chain transfer agent such as hydrogen. Thus, the recycle gas serves to supply the monomer to the bed, to fluidise the bed, and to maintain the bed at the desired temperature. Monomers consumed by the polymerisation reaction are normally replaced by adding make up gas or liquid to the polymerisation zone or reaction loop.
The physical properties of polymer products are well known to those skilled in the art as being dependent on properties such as density, molecular weight and molecular distribution. (Polypropylene and other Polymers Polymerization and Characterization, Ser van der Ven, Studies in Polymer Science 7, Elsevier, 1990). In this paper, branch length is also stated as appearing to play a role in polyethylene properties, not only in the case of melt flow, which in LLDPE behaves in a different manner from other kinds of polyethylenes, but also in more subtle ways. The author illustrates how the ESCR of a polymer increases with increasing branch length from methyl to hexyl.
The author also explains that in terms of strength properties, such as stiffness and yield stress, there exists a unique relation with density for all types of polyethylene including LDPE; stiffness etc. decreasing with increasing density. Density is not a primary independent variable from the polymer synthesis point of view, but not surprisingly the crystallinity is the overriding property determining strength in polymer, and density is a very good measure of this.
In Polymers and Copolymers of Higher alpha Olefins, B. A. Krentsel, Y. V. Kissin, V. J. Kleiner, L L. Stotskaya, Hanser Publishers, 1997, data is presented in Table 8.22 showing how the mechanical modulus measured in Mpa of Ethylene/alpha Olefin Copolymers varies with polymer density and crystallinty. This data shows that the flex modulus behaviour is linear with density in the range 0.92 g/cm3 to 0.96 g/cm3. This is illustrated in FIG. 1.
A polymerisation reactor is typically controlled to achieve a desired melt index and density for the polymer at an optimum production. Beyond density and melt index requirements, it is also very important to achieve a combination of superior polymer properties including e.g. die swell, environmental stress cracking resistance (ESCR), rigidity as measured by either top load resistance or flexural modulus and impact. Consequently, the man skilled in the art is always looking, within the constraints of conventional, safe operation, to improve the way to control the operating parameters of the plant.
It has been found that density measurement and control is not the optimum technique to ensure that the process produces polymers with target die swell, environmental stress cracking resistance (ESCR), rigidity and impact. The present invention provides means to monitor and control stability of the entire polymerisation zone; particularly for high space time yield polymerisation processes. Beyond stability, the present invention provides means to control the process operating parameters in order to achieve the desired combination of superior polymer properties including e.g. die swell, environmental stress cracking resistance (ESCR) rigidity and impact.
According to the present invention, the chain branching level (CBL) has been found to be a simple and effective property in monitoring and maintaining control of polymer properties throughout the polymerisation zone and thereby optimising the stability of the fluidised bed and control of the overall polymerisation process.